1. Field of the Invention
The present invention relates to a magnetic disc driving apparatus, and more particularly to a magnetic disc driving apparatus carrying out tracking control for its magnetic head in combination with open-loop control and closed-loop control.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional magnetic head positioning control system disclosed in, for example, the "Mitsubishi Denkigiho" (Vol. 59, NO.12, 1985, p.14), in which reference 1 designates a speed error detector for detecting an error in the seek speed of a magnetic head, and an output signal B from the speed error detector 1 and an output signal C from a compensator 2 for stabilizing tracking control of the magnetic head are inputted to a control mode switch 3 for switching a control mode to be described below, the control mode switch 3 receiving a control signal D therefor and outputting an output signal E to an actuator (not shown) for the magnetic head.
FIG. 2 is a block diagram of the conventional compensator, in which a compensator 2 comprises a first compensating unit 21 for integrating an input signal A and a second compensating unit 22 for proportionally amplifying the integrated signal.
Next, an operation of the conventional disc driving apparatus will be explained. A seek instruction from a central processing unit, such as a personal computer, starts access of a magnetic head to a target track at a magnetic disc, at which time the head positioning control system is switched by the control mode switch 3 to a mode for controlling the seek speed of the magnetic head called the speed control mode, on the basis of the control signal D. The control mode switch 3 selects the output signal B from the speed error detector 1 and outputs it as the output signal E. In this state, when the magnetic head reaches a preset position on the target track, the control signal D is inverted, the control mode switch 3 selecting the output signal C to output it as the output signal E. The control mode is switched from the speed control mode to the position control mode and, when switched thereto, the magnetic head is tracking-controlled to track the set position of target track at the magnetic disc. Table 1 shows operator of the control mode switch 3 when the control mode is switched. For example, when the control signal D is a level "L", the output signal E=B is obtained and when the same is at a level "H", the output signal E=C is obtained.
TABLE 1 ______________________________________ D E L B H C ______________________________________
The tracking control is always carried out under closed-loop control.
Since the tracking control of the conventional magnetic disc apparatus is carried out under closed-loop control, there is a fear that, when the control mode is switched, the position and speed of the magnetic head may cause the head to jump over the target track at the magnetic head position so as to cause overshoot. When overshoot occurs, it causes the problem that a settling time of the magnetic head becomes longer.